1. Field of the Invention
The present invention relates to devices for delivering feed materials to glass melters. In particular, the present invention relates to distributing or dispersing feed slurries in the form of fine globules for improved melter performance. The United States Government has rights in this invention pursuant to Contract No. DE-AC09-89SR18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
At the Savannah River Site materials to be vitrified are delivered to glass melters in the form of a slurry consisting of finely divided glass frit that is slurried with waste liquids or other solids which are also finely divided. The gel-like slurry feed is delivered to the melter with a pump and is flowed vertically downward through a 0.188 diameter inch tube which is insulated and water cooled. Upon exiting the tube the feed drops, undivided, onto the surface of the melt within an enclosed vessel.
The interior environment of a glass melter is very dynamic thermally. The glass is melted by internal joule heating generated by current supplied by electrodes submerged beneath the surface. Molten glass product is discharged periodically or at a gradual rate to molds, and new glass in the form of slurry feed is added to replace that which is discharged. Some heat is unavoidably lost by the discharge of molten glass. Additionally heat losses also occur through removal of off-gas via the melter plenum. Cooler plenum temperatures cause a crust or "cold cap" to form on the molten glass. A uniform cold cap insulates the molten mass, assists melting, and provides thermal damping necessary for stable melter operation.
The feed to the melter may be modified with additives that improve the glass for use as a waste encapsulating medium. Vitrified waste products can consist of fillers, dopants, coloring agents, additives and waste materials incorporated into a glass matrix by combining the material with glass frit, feeding the slurried combination to the glass melter, melting the glass with the mixed material, and then discharging the composite into a mold. Frequently, however, the slurry feed and sometimes impurities and additives in the feed can further increase the dynamics of the melter environment. The flowing agent in the slurry, usually water, rapidly flashes to steam upon contacting the melt. The waste and other additives may themselves be combustible.
Presently, when feed is dropped in a continuous stream of slurry onto the cold cap, the uniformity of the cap can be disrupted. Often voids result in the cold cap through which heat is lost unnecessarily. This phenomenon is assisted by the rapid combustion and vaporization of organics, water and other volatiles, contained within the feed, upon contact with the hot melt surface. In some instances entrained vapor results in glass foaming within the melter. By distributing the feed evenly in the form of divided droplets across the melt, cold cap integrity is preserved, heat usage is maximized and a greater degree of control is lent to melter operation. In divided form, organics and moisture can be combusted and or vaporized in the melter plenum prior to arrival at the cold cap. This prevents of cold cap disruptions and also reduces direct glass entrainment into the off-gas due to spattering from rapid vaporization at the melt surface.
Additionally, divided feed delivery increases the available surface area of the fed mass during its residence within the plenum. This suggests that existing heaters within the plenum space, which are presently limited in use to providing start-up and trim heating, can now be more productively applied to glass melting. This effect would represent additional heating capacity to that already available by present joule heating and an increase in melter rate capacity is expected.
However, if the feed is too finely atomized, the feed particles themselves can become directly entrained in the off gas and exit the melter through the off-gas system without being melted.
Melter feed devices are also prone to plugging. Plugs readily occur at the exiting tip due to drying out and sinter of the feed material on tip surfaces. The plugs are difficult to remove because they are hard, strong, insoluble, and bond tightly to tip surfaces.
Presently, two strategies for avoiding plugs are employed. The first is to reduce the exit area in combination with feeding to affect an uninterrupted and rapid feed velocity at the tip sufficient to avoid the onset of drying. This method is not effective during periods in which reduced feed rates are dictated by melter dynamics. Additionally, the resulting small orifice size significantly reduces the ability to remove plugs. Plugging under low flow conditions has been experienced even when periodic steam and water purges have been employed. The second strategy is to increase the fraction of water within the feed to reduce the tackiness of the feed and make it less prone to drying. In addition to a higher vapor load to the off-gas system, this method requires more heat per pound of feed, and increases vapor flashing within the melter which results in reduced cap integrity and glass foaming.
A feed implement for a glass melter must be capable of distributing feed in a particle size that is not too small nor too large, to avoid entrainment of feed in the off-gas and enhance cold cap integrity. It must be capable of accommodating glass frit in a slurry, perhaps with a second material or additive carried by the slurry, with low percent water ratios to reduce vapor volatility. It must distribute the feed evenly across the melt surface to maximize cold cap integrity and uniformity. It must function in a dynamic environment without fouling. It must not plug, and it must provide means for removing plugs easily should they occur. Additionally, it must be able to withstand oxidation, sulfudation and creep of components at high temperatures (700 C.) encountered in the melter plenum.
U.S. Pat. No. 4,616,784 issued to Simmons, et al. in 1986 describes a nozzle for atomizing coal-water slurries. A test apparatus utilizing a device as described in this patent was tried in small 100th scale glass melters at the Savannah River Laboratory. However, it atomized the feed into too small of particles, used excessive quantities of air propellant, and did not lend well to scale-up for larger melters, and required significant work and disassembly for correcting plugged and fouled conditions.